1. Field of the Invention
Embodiments described herein relate to semiconductor manufacturing processes. More specifically, embodiments described herein relate to improved methods of forming a doped semiconductor layer on a substrate.
2. Description of the Related Art
Germanium was one of the first materials used for semiconductor applications such as CMOS transistors. Due to vast abundance of silicon compared to germanium, however, silicon has been the overwhelming semiconductor material of choice for CMOS manufacture. Germanium, however, has many properties as a semiconductor that are superior to those of silicon. Germanium has, for example, better electron mobility and lower resistivity under doping than silicon. This has led to renewed interest in germanium as a semiconductive medium for various electronic applications. One such application is logic devices.
Parts of the logic gate where electron mobility is desired include the source, drain, and channel regions of the transistor structure. P-doped and n-doped semiconductor materials are separated by an undoped channel region such that application of an electric field near the channel region causes electrons to flow from the n-doped source to the p-doped drain regions. Good electron mobility encourages good response to the gate voltage. Use of germanium in such regions is therefore preferred over silicon.
In most cases, logic devices are formed on silicon substrates due to the low cost of silicon and its relative structural compatibility with the materials of the logic device and its general processability when forming other devices. When forming a doped germanium layer on silicon, the germanium layer is commonly implanted with dopants, and the resulting layer annealed to activate dopants and repair structural disruption caused by the implantation. Resistivity of the resulting doped material is reduced by incorporation of dopants in the crystal structure and ordering of the matrix.
Epitaxial formation of in-situ doped layers promises to eliminate the structural disruption caused by implantation, because the doped layer is formed without implantation. Epitaxial formation of doped germanium on silicon, however, leaves some residual disorder of the deposited layer due to structural incompatibility of germanium and silicon. The doped layer is typically annealed to remove the residual disorder, but the anneal encourages diffusion of dopants in the germanium layer into the underlying silicon, degrading the electrical properties of the resulting device. A process of forming a doped germanium layer is needed that avoids the unwanted diffusion.